1. Field of the Invention
This invention relates to a fan blade, and more specifically to a replaceable insert on a integrally bladed rotor disk or blisk.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As one skilled in this technology appreciates, the gas turbine power plant typically incorporates fans and compressor blades for imparting compression to intake air (straight jet engines do not incorporate fans) which is then delivered to the combustor where fuel is combusted to add heat thereto. Hence, this engine working medium is accelerated before being delivered to the turbine which is mechanically connected to the fan/compressor so that the energy extracted from the engine working medium is converted to power the compressor fan/blades to pressurize the intake air and develop propulsive thrust. The fan blade, which essentially is a compressor blade that is typically mounted upstream of the smaller compressor blades, not only serve to pressurize the intake air, it also develops thrust which is added to overall thrust developed by the engine.
As will become apparent from the description to follow, this invention is particularly efficacious for use in an Integrally Bladed Rotor (IBR) of a gas turbine engine, but also has potential use for airfoils that are utilized in non-IBR configuration. The following discussion relates to the IBR configuration.
Modern day fan blades are generally highly sophisticated so as to achieve high pressure ratios across the stage(s) of fan blades which require swept airfoils and contoured blade shaping to attain necessary superior aerodynamics. The backward sweep of the airfoil leading edge typically serves to reduce shock losses and noise generation while the forward sweep is typically incorporated to enhance the aerodynamics of the airfoil.
In addition to the aerodynamics of the airfoil, be it a fan, a compressor or a turbine blade, a concern of the designer and the engine user is the stress concentration of the blade. During operation, the blades and disk rotate and generate substantial centrifugal forces which are carried by the disk. The designer of the IBR must assure that the steady tensile stress and the alternating stress are maintained within their limits throughout the entire IBR configuration for the entire operating envelope of the power plant. What is not in the control of the designer is localized stress concentration that is occasioned by blade damage due to foreign object damage (FOD). Ingested objects, such as grit or sand from the runways, small birds, ice, or other objects can damage the blade or the engine. During normal operation, FOD may cause chips or nicks in the blades and most commonly in the leading edge. Obviously, because the IBR is a complicated hardware requiring special materials and expensive manufacturing, it is extremely costly to discard an IBR whenever the blade becomes damaged. Since the IBR is an integral unit of the blades and disks, obviously the removal of the blade for repair purposes is non-existing.
While the teachings disclosed in U.S. Pat. No. 5,725,354 granted to Wadia et al on Mar. 10, 1998 entitled FORWARD SWEPT FAN BLADE, relates to a separate leading edge, the present invention differs not only in the construction thereof, but also the design philosophy. To best understand the difference between these two concepts, one should appreciate the teachings in the Wadia et al reference, supra, where the swept portion of an IBR fan blade is made from a separate composite piece that fits into the portion of the airfoil of the fan blade so as to attain the forward sweep. As is true in all fan blades, the airflow over the blades induces vibration in the airfoil and creates alternating stresses. These stresses, i.e., steady tensile stress and alternating stresses are not uniform and vary over the length, width and thickness of the blade. This situation is acerbated by indications on (damages to) the airfoil such as nicks, chips, cracks and the like caused by sand, dirt, bird ingestion, or other objects. These cause stress concentration. The problem is more complicated when the leading edge is significantly swept forward. While this innovation provides aerodynamic benefits, it changes the center of gravity of the blade and hence, complicates the stress design of the airfoil. Hence, the purpose of the teachings of the Wadia et al '354 patent, supra, is to provide a design configuration that improves the stress problem by reducing the steady stress along the leading edge of the airfoil which would otherwise occur from effecting forward sweep.
Of significance, the present invention differs from the Wadia et al '354 patent and other known prior art designs by 1) providing a removable leading edge that can be fabricated from the same or different materials, and 2) designing the leading edge as an independent load carrying member where it includes an airfoil portion and an attachment portion (root), such that the loads on the airfoil are transmitted from the airfoil through the root of the leading edge into the disk. Hence, the disk, whether an IBR or non-IBR fans, must be designed to accommodate the root of the leading edge for attachment thereto.
In addition, foreign object damage (or, FOD) is a significant and continuing problem for both military and commercial aviation engines. FOD is estimated by Boeing to cost the aerospace industry some $4 billion per year and estimates the cost to repair a FOD-damaged engine to easily exceed $1 million. The cost of repairing FOD damage to an engine can easily exceed 20% of the engine's original purchase price.
Repair of IBRs that are now common to modern military aircraft engines present a real concern for maintenance in the event of FOD. The lower aspect ratios of modern fan blades (resulting in blades that resemble heavy meat cleavers rather than long carving knives) should provide greater tolerance to FOD and bird strike. However, if IBR blades are damaged, their repair may be more difficult because the blades cannot be replaced on the flight line (airports, aircraft carriers, air bases, and other places of operation). Although ‘on-wing blending’ (filing a fan blade's damaged leading edge, within tolerable limits, without removing the engine from the aircraft) may be an acceptable repair technique for some minor dents, more serious damage will likely result in the engine having to be removed, and entire IBRs will need to be replaced or replacement blades welded on.
Another prior art reference, U.S. Pat. No. 4,342,542 issued to Tan et al on Aug. 3, 1982 and entitled MOVING BLADE OF STEAM TURBINE discloses a rotor blade with a leading edge with an anti-erosion plate attached by welding that extends from the blade tip to almost the platform. One problem with using this type of replaceable leading edge for a modern IBR is that the surface area on which the bond is formed between the replaceable leading edge insert and the blade is small resulting in higher stresses occurring on the bond. Also, the bond is required to secure the insert to the blade against all the directions including the radial direction which opposes the centrifugal force developed due to rotation. Another problem is the insert is not easy to replace, especially in the field of usage (e.g., at a machine shop).
U.S. Pat. No. 5,785,498 issued to Quinn et al on Jul. 28, 1998 and entitled COMPOSITE FANBLADE TRAILING EDGE REINFORCEMENT discloses a fan blade with a replaceable insert for a top portion of the trailing edge of the blade, where in the embodiment of FIG. 3 of this patent shows a single mechanical fastener (a nut and bolt) that further secures the insert to the blade along with a bonding substance. One problem with using this type of replaceable leading edge for a modern IBR is that the insert does not cover the leading edge for overcoming the FOD problem and the insert does not extend along most of the edge of the blade, leaving a large portion of the edge exposed to damage on a non-replaceable surface.
An object of the present invention is to provide for an airfoil of a compressor or fan for a gas turbine engine of the IBR type a detachable leading edge that can be easily removed and replaced.
Another object of the present invention is to provide for an IBR leading edge that is secured to a blade by a disbondable composition that will not be released during a lightening strike.
Another object of the present invention is to provide for an IBR leading edge that that is secured to a blade by a disbondable composition that will act to hold the insert against radial displacement with respect to the blade during operation.
Another object of the present invention is to provide for an IBR leading edge that is secured to the blade along the entire length to prevent unzipping of the insert from the blade from FOD.
Another object of the present invention is to provide for a blade retainer ring that will balance the rotor disk as well as retain a shear pin within slots.